Field of the Invention
The present disclosure relates in general to a wedge locking washer. Furthermore, it relates to a method of producing a wedge locking washer comprising cold forming and surface hardening.
Description of the Related Art
A wedge locking washer is a thin disk-shaped plate with a central hole adapted for a screw shank that is able to prevent undesired untightening of a screw joint. A wedge locking washer is used to distribute the load of a threaded fastener. This application deals with wedge locking washers which are generally used in pairs. On one of the surface sides there is provided a pattern in the form of radially extending cams, also known as wedges. In a locking system, two wedge locking washers are arranged in a pair with the cam sides facing and engaging each other. The largest surface of the cams has an inclination, in relation to a horizontal plane, which is larger than the pitch of the threads of a screw in such a locking system. The inclination causes a positive and efficient locking of the fastening element by creating a wedge effect by the cams which in turn prevents a bolt or the like from rotating loose even when subjected to vibrations. This is thus an effect that is achieved when the two wedge locking washers rotates in relation to each other in such a way that the cams slide upon each other. When this sliding of the cams occurs the washer pair expands in axial direction, as the cams slide upon each other. If tightening of a joint instead is desired there is no corresponding expansion of the two washers as the sliding of the cams is prevented by the end of the wedge shape.
The surface opposite the cam surface of the wedge locking washer is often provided with another pattern adapted to increase the friction between the wedge locking washer and a screw head, a workpiece, a bolt or the like. Such a pattern may for example be in the form or radially extending teeth. Each tooth typically has a shorter peripheral extension than a peripheral extension of a cam on the opposite surface side of the wedge locking washer and is inclined in the opposite rotational direction to the inclination of the cams. The reason for this is that it should be possible for the teeth to interact and engage with a surface of for example a screw head, nut or an element that is attached, such that the washer is prevented from rotation with regard to this surface. Instead of a pattern other friction increasing surface treatments can be applied. Such treatments use of particles of hard metal applied to the surface.
Locking washers are produced from a strip blank of an appropriate material. The strip blank is subjected to forming and/or punching in order to produce the shape and surface pattern of the wedge locking washer. On example of a way of forming a wedge locking washer is disclosed in EP2195129. Wedge locking washers are required to have a high surface hardness in order to withstand the forces and in order not to deform the patterns of the teeth and cams such that they would lose their function in a locking system. Furthermore, a high surface hardness is desirable from a wear perspective. Therefore, it is appropriate to harden a material of a wedge locking washer in order to achieve the desired hardness. Depending on the material of the wedge locking washer, it may additionally or alternatively be appropriate to provide a hardened surface of the wedge locking washer or a hard coating to achieve the desired properties.
Wedge locking washers can for example be made of austenitic stainless steels such as AISI 304, AISI316 or AISI1316L. Austenitic stainless steels have a fairly high strength but cannot be through hardened. Austenitic stainless steels may therefore be susceptible to wear and should therefore be surface hardened in order to increase the surface hardness and reduce wear. Surface hardening of austenitic stainless steels can be achieved through various methods, such as to nitride the steel by plasma or by salt bath nitriding. Such methods may however reduce the corrosion resistance due to formation of chromium nitride in the surface layer. However, there are suitable options for surface hardening in which the corrosion resistance is maintained or even improved. Such surface hardening processes are in general based on subjecting the material to a gaseous thermochemical process. Such a process involves diffusion of carbon and/or nitrogen from a surrounding gas into the surface of the steel resulting in a surface layer enriched in carbon and/or nitrogen. In contrast to plasma nitriding or salt bath nitriding, such diffusion processes do not result in formation of chromium nitride but the nitrogen and carbon is provided interstitially into the microstructure. Hardening is performed after a part has been formed into its final shape, i.e. when no additional plastic deformation of the steel in order to achieve the desired final geometrical shape is intended.
As stated above, the diffusion of carbon and/or nitrogen into the surface may also increase the corrosion resistance of the austenitic stainless steel. This is however only possible in case there is no delta ferrite or deformation martensite (also known as deformation induced martensite) on the surface or in the surface region of the steel. Furthermore, the surface should be essentially free from surface defects in order to achieve the desired result. Otherwise, the risk for localised corrosion may occur.
Deformation martensite may arise when producing products using for example cold forming, deep drawing, stamping or pressing. Deformation martensite negatively influences the corrosion resistance of austenitic stainless steels not only during the surface hardening as disclosed above but also in general. It is possible to remove deformation martensite by solution annealing. However, such solution annealing also influence the bulk hardness and is therefore not suitable for all applications.
As disclosed above, wedge locking washers are formed by cold forming, such as punching or stamping, from a sheet blank of the steel. Austenitic stainless steels generally possess high plasticity and can easily be formed by cold forming. However, during cold forming deformation martensite may form as a result of the deformation introduced into the material. In view of the fact that the degree of deformation varies as a result of the patterns of the wedge locking washers, the deformation maritensite generally occurs more often in the most deformed portions of the wedge locking washer.
Delta ferrite may be present in the steel as a result of the manufacturing process. Delta ferrite also has a negative effect on corrosion resistance in general.
One way of improving the corrosion resistance of an austenitic stainless steel is by electro-polishing the surface of steel parts in order to dissolve contaminations and deformation martensite, as well as obtaining a smoother surface free from cavities in which it can be difficult to build up a passivating surface layer necessary to obtain the intended corrosion resistance. The electro-polishing also generally result in a higher content of chromium at the surface.
Electro-polishing is a process typically comprising immersing the metal part to be electro-polished in an electrolyte and allowing the metal part to act as an anode. Metal on the surface of the metal part to be electro-polished is oxidised and dissolved in the electrolyte as a current passes from the anode to the cathode. Electro-polishing of a surface hardened austenitic stainless steel thus removes a portion of the surface hardened layer of the austenitic stainless steel which is undesirable.
Electro-polishing of metal parts having complex geometries, such as sharp edges or deep holes, may however result in an uneven removal of metal from the surface of the part. This is due to different current densities at different parts of the surface resulting from the geometry of the metal part. Thus, electro-polishing of previously known wedge locking washers may cause different amount of metal removed from the edge of the cams and from the bottom of the cams.